1. Field of the Invention
The present invention relates to a zoom lens suitable to a still camera, a video camera, and a digital still camera and an image pickup apparatus having the same.
2. Description of the Related Art
In recent years, an image pickup apparatus (camera) using a solid-state image sensor such as a video camera or a digital still camera is more and more small-sized and has a high function. In this regard, it is desired that in a photographic optical system used in the image pickup apparatus like this, the total size of a lens system is small and that the zoom lens has a high optical performance.
In a camera like this, various optical members such as a low-pass filter or a color correction filter are disposed between a lens rearmost portion and an image sensor. Accordingly, it is desired that the zoom lens used in such an image pickup apparatus has a relatively long back focus.
Furthermore, it is desired that in a camera using an image sensor for a color image, chromatic aberration of magnification is effectively corrected. Furthermore, it is desired that a zoom lens used in such an image pickup apparatus has a high telecentric performance on an image side to prevent color shading.
In this regard, U.S. Pat. No. 6,498,687 discusses a three-unit zoom lens including, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power, which is small-sized and has a high telecentricity on the image side.
In addition, U.S. Pat. No. 4,838,666 discusses a three-unit zoom lens of the above-described zoom type. The three-unit zoom lens of the above-described zoom type has a long back focus, a high telecentric performance on the image side, and a wide angle of view. In this regard, U.S. Pat. No. 6,038,084 discusses a three-unit zoom lens of the above-described zoom type having a zoom ratio as high as 3 or higher.
Furthermore, in recent years, light transmissive ceramic has been developed and a photographing optical system that uses light transmissive ceramic as a material of an optical member is conventionally used. In this regard, the light transmissive ceramic has a refractive index higher than that of optical glass. In addition, the hardness and the strength of light transmissive ceramic are higher than those of optical glass. A conventional image pickup apparatus utilizes these characteristics of the light transmissive ceramic and thereby reduces the total thickness of a lens system thereof.
Meanwhile, a so-called short zoom type two-unit zoom lens having a wide angle of view has been conventionally used, which is constituted by two lens units, namely, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, and changes the interval between the first and the second lens units to perform magnification. In this regard, U.S. Pat. No. 7,054,072 discusses a two-unit zoom lens that uses the above-described the light transmissive ceramic as a material of a negative lens in the second lens unit. Thus, the zoom lens discussed in U.S. Pat. No. 7,054,072 reduces the total size its lens system.
In recent years, it is desired that a zoom lens used in an image pickup apparatus has a high zoom ratio and the total size of the lens system is small. Generally, in order to reduce the size of a zoom lens, the number of lenses is reduced while intensifying the refractive power of each lens unit constituting the zoom lens.
However, if the number of lenses is reduced while intensifying the refractive power of each lens unit, the lens thickness increases. Accordingly, in this case, the size of the lens system cannot be appropriately reduced. Furthermore, in this case, it becomes difficult to effectively correct various aberrations. Accordingly, in order to reduce the total size of a lens system while increasing the zoom ratio, it is significant to appropriately set the zoom type, the refractive power of each lens unit, and the configuration of each lens constituting each lens unit.
In addition, in order to achieve a zoom lens having a high zoom ratio and whose lens system is small-sized, it is also significant to select the material of each lens constituting the lens unit by appropriately setting the refractive power and the Abbe number so that various aberrations such as chromatic aberration of magnification occurring at each zoom position can be suppressed or reduced. The refractive index, the hardness, and the flexural strength of the above-described light transmissive ceramic are higher than those of ordinary optical glass. It is useful in effectively correcting aberrations and reducing the total size of an optical system to use the light transmissive ceramic having the above-described characteristics as an optical material.
However, in the case where a lens made of such light transmissive ceramic is merely used in a zoom lens, the zoom lens may not have a high optical performance for the entire zoom range. In order to achieve a high zoom ratio while reducing the total size of a lens system with a lens made of the above-described light transmissive ceramic, it is significant to appropriately set the zoom type, the refractive power of each lens unit, and the configuration of the lens in each lens unit.
On the other hand, in the case where the zoom ratio of a zoom lens is increased and thus a focal length at a telephoto end becomes long, the material of the lens may expand due to the change (rise) in the temperature. Accordingly, in this case, a phenomenon of defocusing may occur and thus the optical performance of the zoom lens may degrade. Accordingly, with respect to the above-described three-unit zoom lens, in order to reduce the total size of the lens system and to achieve a zoom lens having a wide angle of view and a high zoom ratio, it is significant to appropriately set the configuration of the lens in each lens unit and to use an optimal material of the lens in each lens unit.
In this regard, if the configuration of the lens in a second or a third lens unit is inappropriately set or if an inappropriate material is used as a material of the lens in the second or the third lens unit, then the aberration may greatly vary during zooming. In this case, it becomes difficult to achieve a high optical performance for the entire zoom range.